Understanding the Impact of Halogenation on the Crystalline Photomechanical Response Properties of 9-Anthracene Carboxylic Acid from First-Principles

Photomechanical organic crystals transform light into mechanical work via solid-state photochemical reactions. While many examples of photomechanical systems have been demonstrated, significant gaps in understanding the relationships between the chemical structure of the photochrome molecule, its crystal structure, and the observed photomechanical response remain. Focusing on the case of [4 + 4] photodimerization of 9-anthracene carboxylic acid, this density functional theory study examines the impact of replacing hydrogen atoms with fluorine or chlorine in various positions on the photomechanical response. The results demonstrate how the interplay of intramolecular deformations and intermolecular crystal packing interactions means that even simple chemical substitutions that preserve the overall crystal packing motif can impact the photochemical response in ways that are not obvious a priori. In addition, the changes in work density obtained from the halogenated species here are an order of magnitude smaller than what previous work found can be obtained through varying the crystal packing motif. This suggests that crystal engineering should take precedence when trying to increase the work density of a system, while additional minor molecular modifications can be used to refine the photomechanical response further or to tune other properties of the material.